When a rubber article is vulcanized, it goes through a soft tacky stage characteristic of unvulcanized rubber to a tack free tough stage characteristic of fully vulcanized rubber. If heat is applied after the optimum vulcanization is reached, the rubber will begin to exhibit what is commonly known as reversion. Once rubber has begun to revert, many of its desirable properties are lost. Reversion can occur because of excess heat applied during vulcanization. Because of the relatively low heat conductivity of rubber, thick rubber articles, such as large tires, are normally vulcanized for a period of time far in excess of the time required to vulcanize the surface of the article. This excess exposure to high temperature causes the properties of the rubber close to the surface to deteriorate. Thin rubber articles, such as rubber bands, can also experience reversion because high vulcanization temperatures are used in order to maximize the production capacity.
Reversion of rubber can also occur during the use of rubber articles. When rubber articles are flexed, heat is generated internally in the rubber. This generated heat can be of such magnitude as to cause reversion. This occurs quite frequently in large tires, such as truck, bus, airplane, and off-the-road tires.
Several synthetic elastomers have excellent reversion resistance, such as EPDM, butyl, and the like. These elastomers, although excellent for reversion resistance, have high heat buildup properties, which preclude their use in certain areas of articles such as large tires.
Natural rubber and its synthetic counterpart cis-polyisoprene are excellent elastomers for low heat buildup properties and for this reason, these elastomers are used quite extensively in thick rubber articles that are subjected to severe flexing, such as large tires. Although natural rubber has low heat buildup properties, unfortunately, natural rubber also has a greater tendency to revert than does other general purpose elastomers, such as styrene-butadiene, cis-polybutadiene and the like.